Pulse width controller

ABSTRACT

A pulse width controller for a thermal processing system is disclosed. Pulsed electromagnetic radiation is directed through a rotatable wave plate to a polarizing beam splitter, which reflects and transmits according to the phase angle of the wave plate. Radiation transmitted by the polarizing beam splitter is directed into an optical circuit that returns the radiation to the polarizing beam splitter after a transit time. A second rotatable wave plate is positioned in the optical circuit. The polarizing beam splitter reflects and transmits the returned radiation according to the phase angle of the second rotatable wave plate. A second pulse width controller may be nested in the optical circuit, and any number of pulse width controllers may be nested.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationSer. No. 61/703,487, filed Sep. 20, 2012, which is incorporated hereinby reference.

FIELD

Embodiments described herein relate to apparatus and methods of laserthermal processing. More specifically, apparatus and methods describedherein relate to a pulse width controller incorporated in a laserthermal processing apparatus.

DESCRIPTION OF THE RELATED ART

Thermal processing is commonly practiced in the semiconductor industry.Semiconductor substrates are subjected to thermal processing in thecontext of many transformations, including doping, activation, andannealing of gate source, drain, and channel structures, siliciding,crystallization, oxidation, and the like. Over the years, techniques ofthermal processing have progressed from simple furnace baking, tovarious forms of increasingly rapid thermal processing such as RTP,spike annealing, and laser annealing.

Conventional laser annealing processes use laser emitters that may besemiconductor or solid state lasers with optics that focus, defocus, orvariously image the laser light into a desired shape. A common approachis to image the laser light into a line or thin rectangle image. Thelaser light is scanned across a fixed substrate or the substrate isscanned beneath the laser light to process the entire surface of thesubstrate.

As device geometry continues to decline, semiconductor manufacturingprocesses such as thermal processing are challenged to develop increasedprecision. In many instances, pulsed laser processes are being exploredto reduce overall thermal budget and reduce depth and duration of energyexposure at the substrate. Challenges remain, however, in creating laserpulses having a temporal shape that affords the desired processingperformance, with the uniformity needed for uniform processing acrossthe surface of a substrate. Thus, there is a continuing need forapparatus and methods of adjusting the temporal shape of an energypulse.

SUMMARY OF THE INVENTION

A optical system for controlling width of an energy pulse, and anapparatus including such an optical system, are disclosed. The opticalsystem features a rotatable wave plate that outputs oriented radiationto a polarizing beam splitter. Radiation transmitted by the polarizingbeam splitter is routed through an optical circuit that has a secondrotatable wave plate. Radiation from the second rotatable wave plate isrouted back to the polarizing beam splitter along the axis of lightoriginally reflected by the polarizing beam splitter. The firstrotatable wave plate is rotated to control the fraction of radiationthat enters the optical circuit, and the second rotatable wave plate isrotated to control the fraction of radiation in the optical circuit thatexits through the polarizing beam splitter.

Multiple optical circuits may be nested in and/or pendant from a firstoptical circuit, each with its input and output optical gates. Opticaldelay legs may also be used in some embodiments to add further delaycomponents.

The optical system described above may be included in a thermalprocessing apparatus featuring an energy source that produces radiantenergy in pulses or continuous wave. The energy that has been routedthrough the pulse width controller is directed to an optical system foradjusting the spatial or temporal profile of the energy, such as ahomogenizer, etalon, or fiber bundle. The energy may then be directed toan aperture to trim non-uniform edges, and then to a substrate tothermally process the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a schematic diagram of a pulse width controller according toone embodiment.

FIG. 2 is a schematic diagram of a pulse width controller according toanother embodiment.

FIG. 3 is a schematic diagram of a pulse width controller according toanother embodiment.

FIG. 4 is a plan view of a thermal processing system according toanother embodiment.

FIG. 5 is a graph showing pulse intensity as a function of time for anenergy pulse processed using apparatus and methods of FIG. 1-4.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a pulse width controller 100 accordingto one embodiment. The pulse width controller 100 has a first rotatablewave plate 102, comprising a first wave plate 114 and a first actuator116, optically coupled to a polarizing beam splitter 104. The polarizingbeam splitter 104 has an optically active surface 106 that reflects aportion of incoming electromagnetic radiation along a reflecting axis126 and transmits a portion of the incoming electromagnetic radiationalong a transmitting axis 128. The position of the first wave plate 114,as rotated by the first actuator 116, determines the polarization axisof the radiation incident upon the polarizing beam splitter 104, and theangle of the polarization axis of the incident radiation, compared withthe polarizing angle of the optical surface 106, determines the degreeof reflection and transmission.

The transmitted radiation 128 is routed to an optical circuit 136 thatdelivers a return radiation 134 to the polarizing beam splitter alongthe reflecting axis 126. At least a first reflector 108 and a secondreflector 110 make up the optical circuit 136. The first reflector 108is disposed along the transmitting axis 128 of the polarizing beamsplitter 104, faces a first deflection direction 138, and propagates afirst deflected radiation along a first deflection axis 130. The secondreflector 110 is positioned to receive radiation originating from thefirst reflector 108, faces a second deflection direction 140, andpropagates a second deflected radiation 132 along the reflecting axis126 of the polarizing beam splitter 104. This may be done using only tworeflectors in some embodiments, while in other embodiments morereflectors may be disposed in the optical circuit. In one embodiment, athird reflector (not shown) is positioned along the first deflectionaxis 130 and propagates radiation along a third deflection direction tothe second reflector, with the angles of the reflectors suitablyadjusted to align the radiation along the reflecting axis 126.

A second wave plate 118 is disposed in the optical circuit 136 at anydesired point. The second wave plate 118 may be disposed along thetransmitting axis 128 between the polarizing beam splitter 104 and thefirst reflector 108, along the first deflection axis 130, along thereflecting axis 126 between the polarizing beam splitter 104 and thesecond reflector 110, or at any position in the optical circuit 136between the first reflector 108 and the second reflector 110. The secondwave plate 118 may be a second rotatable wave plate 112, comprising thesecond wave plate 118 and a second actuator 120, if desired.

The second wave plate 118 rotates the polarization axis of the seconddeflected radiation 132 by 90°, such that the return radiation 134 willpass through the optically active surface 106 of the polarizing beamsplitter 104. The return radiation 134 thus propagates along thereflecting axis 126 slightly later than the radiation originallyreflected along the reflecting axis 126 by the transit time of theoptical circuit 136. If the transit time of the optical circuit 136 isless than the time duration of the incident pulse, the radiationreflected along the reflection axis 126 and the return radiation 134will overlap to form a single extended pulse. The extent of thebroadening may be controlled by setting the first rotatable wave plate102. If a difference between the polarization angle of the firstrotatable wave plate 102 and the polarization angle of the opticallyactive surface 106 approaches 90°, most incident radiation will bereflected along the reflection axis 126, and the resulting pulse will bebroadened only slightly with a decay tail added by the circulatedportion of the incident pulse. If the difference is nearly zero, theresulting pulse will be substantially delayed by the circuit transittime and slightly broadened with a rise tail created by the initiallyreflected radiation. If the difference is substantially far from eitherextreme (0° or 90°), the resulting pulse will be broadened and will bebimodal, with the intensities of the two peaks determined by the phasedifference.

The second wave plate 118 may have a polarization angle that does notalign with the polarization angle of the optically active surface 106.In such an embodiment, the return radiation 134 will be partiallytransmitted and partially reflected at the optically active surface 106,depending on the phase angle difference between the wave plate 118polarization angle and the polarization angle of the optically activesurface 106. A plurality of pulses will then emerge as the radiationcirculates around the optical circuit 136, and the intensity of eachpulse will be a function of the angles of the first wave plate 114 andthe second wave plate 118. The second wave plate 118 may be coupled to asecond actuator 120 to form a second rotatable wave plate 112, so theangle of the second wave plate 118 may be adjusted.

By varying the settings of the two rotatable wave plates 102 and 112, apulse of electromagnetic radiation incident on the pulse widthcontroller may be divided into two or more pulses propagating along thereflecting axis 126. The pulse width of the incident pulse may beeffectively controlled by the pulse width controller 100 if the transittime of the optical circuit 136 is substantially less than the timeduration of the incident pulse. For example, for an 8 nsec energy pulse,an optical circuit having a transit time substantially less than 8 nsec,such as a total length less than about 240 cm, for example a totallength between about 100 cm and about 200 cm, will afford a single pulsehaving a tunable pulse width. A controller 122 may be coupled to theactuators 116 and 120 to control the relative phase angles of the twowave plates 114 and 118.

Actuators may be deployed to adjust the length of the optical circuit136. In the embodiment of FIG. 1, a first actuator 144 is coupled to thefirst reflector 108 and a second actuator 142 is coupled to the secondreflector 110. Each of the first actuator 144 and the second actuator142 is coupled to the controller 122, and the controller 122 isprogrammed to adjust the positions of the first and second reflectors108 and 110 to change the length of the optical circuit 136 whilemaintaining alignment of the first and second reflectors 108 and 110. Inthe embodiment of FIG. 1, each of the first and second actuators 144 and142 may be a linear and rotational actuator, or one of the firstactuator 144 or the second actuator 142 may be a linear and rotationalactuator while the other is a rotational actuator only. In analternative embodiment wherein three reflectors are included in arectangular optical circuit, as described above, two adjacent reflectorsof the optical circuit may be coupled to a support that is moved by alinear actuator to adjust the length of the optical circuit.

Adjusting the length of the optical circuit may provide fine controlover the temporal profile and width of a combined pulse. The controller122 may be programmed to control an energy source to produce pulses 124having a desired duration and periodicity, to control rotation of thewave plates 102 and 112 to control replication and splitting of thepulses, and to control the length of the optical circuit 136 using theactuators 144 and 142 to produce a wide variety of shaped energy pulses.In some embodiments, the pulse width controller 100 may alter thefrequency of pulses in a pulse train, for example by doubling thefrequency. In such embodiments, pulse amplitude is typically lowered byinteraction with the pulse width controller 100 if the optical circuit136 has a transit time less than the periodicity of the pulses. However,if the transit time of the optical circuit 136 is greater than theperiodicity of the pulses, the pulse width controller 100 may be tunedto function as a pulse amplifier by delaying pulses such that a firstpulse travels through the optical circuit 136 and emerges inco-propagating relation to a second incident pulse partially reflectedby the polarizing beam splitter 106.

FIG. 2 is a schematic diagram of a pulse width controller 200 accordingto another embodiment. The pulse width controller 200 features many ofthe same components as the pulse width controller 100. In the embodimentof FIG. 2, however, the pulse width controller 200 has a first opticalcircuit 202 and a second optical circuit 208 nested within the firstoptical circuit 202. The rotatable wave plate 102 and the polarizingbeam splitter 104 control radiation admitted into the first opticalcircuit 202, as with the pulse width controller 100 of FIG. 1, but asecond polarizing beam splitter 206 is positioned between the firstreflector 108 and the second reflector 110 of the first optical circuit202. The second polarizing beam splitter 206 reflects a portion ofincident electromagnetic radiation along a second reflecting axis 216and transmits the remainder along a second transmitting axis 218, thefraction reflected and transmitted being dependent on the relativepolarization angles of the second polarizing beam splitter 206 and athird rotatable wave plate 204 which, in alternate embodiments, may benon-rotatable.

The transmitted radiation is circulated around the second opticalcircuit 208 by a third reflector 214 and a fourth reflector 212 andthrough a fourth wave plate 210, which may be a rotatable wave plate, asdescribed above. There may be, of course, more than two reflectors inthe second optical circuit 208.

The second optical circuit 208 further broadens the radiationpropagating within the first optical circuit 206 according to similareffects. If the transit time of the second optical circuit 208 is lessthan the time duration of the energy pulse incident on the secondpolarizing beam splitter 206, the radiation that propagates along thesecond reflecting axis 216 will propagate as a broadened pulse, whichwill further broaden the radiation ultimately propagating along thefirst reflecting axis. The four wave plates 102, 112, 204, and 210, maybe independently controlled to produce a radiation pulse along the firstreflecting axis 126 that has a much broader range of temporal shapes anddurations than are available with a single optical circuit. Pulse widthcontrol circuits may be nested, as the second optical circuit 208 isnested within the first optical circuit 202, to any desired depth.Alternately, or additionally, pulse width control circuits such as thesecond optical circuit 208 may be proliferated in series around thefirst optical circuit 202. Any combination of serial and nested pulsewidth control circuits may be employed to achieve a desired control overpulse width, and all actuated wave plates may be controlled by acontroller to provide precise control over the pulse shape and duration.

The pulse width controller 200 has an actuator 222 coupled to a support220 for moving portions of the first optical circuit 202 and all of thesecond optical circuit 208 to control overall transit time of theoptical circuit 202. Because the optical circuit 202 is arrangedaccording to a rectilinear configuration, the actuator 222 may adjustthe length of the optical circuit 202 by moving coaxial components ofthe optical circuit 202 in a direction perpendicular to their commonoptical axis. The second optical circuit 208 is coupled to the support220 to maintain alignment with the actuated components of the firstoptical circuit 202. In the embodiment of FIG. 2, the first reflector108, the third reflector 212, the fourth reflector 214, the third waveplate 204, the fourth wave plate 210, and the second polarizing beamsplitter 206 are all coupled to the support 220 and the actuator 222adjusts a distance between the first polarizing beam splitter 104 andthe first reflector 108 and a distance between the second polarizingbeam splitter 206 and the second reflector 110 to adjust length of theoptical circuit 202. Naturally, an alternate embodiment may couple thesecond reflector 110 and all components of the second optical circuit208 to a support to adjust a distance between the third wave plate 204and the second polarizing beam splitter 206 and a distance between thesecond reflector 110 and the second wave plate 112 to adjust length ofthe optical circuit 202. A controller 230 is coupled to the wave plates102, 112, 204, and 210, and to the actuator 222, to control theperformance of the pulse width controller 200.

FIG. 3 is a schematic diagram of a pulse width controller 300 accordingto another embodiment. The pulse width controller 300 of FIG. 3 featuresthe same entrance regulating features, the rotatable wave plate 102 andthe polarizing beam splitter 104, with a different optical circuit 302.The optical circuit 302 features delay legs 304 interposed along theoptical circuit 302 to add transit time and subdivisions to the opticalcircuit 302, if desired. Each delay leg 304 features a partial reflector306 and a full reflector 308, and one delay leg 304 may be opticallycoupled to another delay leg 304 to increase the pulse-broadening effectof the optical circuit 302. The pulse width controller 300 has fourdelay legs 304A, 304B, 304C, and 304D. Radiation incident at partialreflector 306A is partially reflected toward the partial reflector 306Dand partially transmitted toward the partial reflector 306B. Radiationincident at partial reflector 306B is partially reflected toward partialreflector 306C and partially transmitted toward two full reflectors 310and 312 that direct the radiation around to partial reflector 306C. Theradiation incident at partial reflector 306C from full reflector 312 ispartially reflected toward full reflector 304C and partially transmittedtoward the partial reflector 306D. Radiation reflected toward the fullreflector 304C is reflected back toward the partial reflector 306C,which subdivides the radiation further. The radiation circulating andcounter-circulating within the optical circuit 302 is attenuated into asmeared-out pulse that is subjected to the polarization angle of thewave plate 314, which may be a rotatable wave plate similar to therotatable wave plate 102, producing the same transmission/recirculationeffect at the polarizing beam splitter 104.

The delay legs 304 introduce transit time and counter-circulation to theoptical circuit 302 that is not present in the optical circuits 136,202, and 208, but at the expense of some energy loss. The variousreflective and refractive surfaces scatter a small amount of incidentradiation, so the combined effect of many delay legs may result in powerlosses that are more than desired. It should be noted that delay legs,such as the delay legs 304 may be used in combination with auxiliaryoptical circuits such as those described above in connection with FIG.2. For example, a rotatable wave plate and polarizing beam splitter maybe positioned along the optical axis between the full reflector 310 andthe full reflector 312 to anchor an optical circuit such as the opticalcircuit 208, if desired. The combination of optical circuits and delaylegs may provide expanded options for tailoring of pulse widths andenergy profiles.

FIG. 4 is a plan view of a thermal processing apparatus 400 according toanother embodiment. A radiant energy source 402 produces a directedradiant energy field that propagates along a first optical axis 414. Thedirected radiant energy field enters a pulse width controller such asthe pulse width controller 100, and emerges along a second optical axis416 having a tailored temporal profile, as described above. The pulsewidth controllers 200 and 300, and other embodiments of pulse widthcontrollers described herein, may also be used.

The radiant energy emerging from the pulse width controller 100 entersan optical system 404 that further shapes the radiant energy fieldaccording to the needs of the application. The optical system 404 mayfeature lenses, filters, prisms, partial and total reflectors, such asmirrors and retroreflectors, etalons, fiber optics, and similarcomponents, to transform the radiant energy field in particular ways.The optical system 404 may feature one or more arrays of lenses thatoverlap portions of the radiant energy field to form a blended, orhomogenized, image with reduced spatial variation. The optical system404 may also feature differential delay optics such as fiber bundles andetalons to reduce coherence in the radiant energy field. Thedifferential delay optics may also be effective to reduce variation inthe temporal profile of the energy field emerging from the pulse widthcontroller 100.

The radiant energy emerges from the optical system 404 along a thirdoptical axis 418 to encounter an aperture 406. The aperture 406 trimsthe radiant energy field to a desired shape and removes edgenonuniformities of the radiant energy field. The resulting energyemerges along a fourth optical axis 420, and is directed toward asubstrate support 410 by any suitable steering optic 408, such as amirror, or any system of reflective and refractive optics for directingthe radiant energy toward the substrate support 410. The radiant energygenerally approaches the substrate support 410 along a fifth opticalaxis 422, which may be generally perpendicular to the plane of asubstrate support surface 424 of the substrate support 410, or may beinclined at a desired angle. The angle of incidence of the fifth opticalaxis 422 with respect to the substrate support surface 424 is typicallybetween about 85° and about 90°, usually about 90°.

A substrate positioned on the substrate support 410 is subjected to theradiant energy for thermal or optical processing. If the radiant energyfield does not cover the entire substrate, the substrate support 410 maybe movable in a plane defined by the substrate support surface 424 ofthe substrate support 410. A precision x-y stage may be used for suchpurposes. A first treatment zone may be positioned in the path of thefifth optical axis 422, and after processing the substrate may be movedin the plane parallel to the substrate support surface 424 such that asecond treatment zone is positioned in the path of the fifth opticalaxis 422 for processing. This process may be repeated until all desiredareas of the substrate are processed. It should be noted that, althoughFIG. 4 depicts a substrate resting on a substrate support and facingupward toward the fifth optical axis 422, the substrate may be orientedvertically, or substantially vertically, and may be positioned above thefifth optical axis 422. The substrate support 410 may be oriented at anydesired angle.

In one aspect, the energy source 402 may be a pulsed energy source or acontinuous wave energy source. The energy source 402 may emit radiantenergy having any desired coherence and any desired frequency. The pulsewidth control methods and apparatus described herein are notsubstantially different for radiation having a broad spectral range,spectral distribution range, and coherency range. Continuous wave andpulse lasers may be used, individually or in combination, to producedesired temporal profiles of pulsed energy having any spectralcharacteristics or combination thereof.

The apparatus described herein are also embodiments of novel methods.The temporal profile of an incident pulse of radiant energy may beeffectively controlled by splitting the pulse into a first sub-pulse anda second sub-pulse using differential polarity, routing the secondsub-pulse through an optical circuit to delay propagation of the secondsub-pulse relative to the first sub-pulse, and releasing the secondsub-pulse after a delay to propagate along the same axis as the firstsub-pulse. If the delay of the optical circuit is substantially lessthan a duration of the incident pulse, the two sub-pulses emerge in anoverlapping temporal relationship, effectively producing a broadenedpulse. The optical circuit is constructed using reflectors that routethe second sub-pulse around a polygonal path so that the secondsub-pulse returns to the location at which it was divided from the firstsub-pulse.

The differential polarity may be provided using a wave plate that isrotated to control the polarization angle of the incident pulse. Theincident pulse is directed to a polarizing beam splitter that reflects aportion of the incident pulse, depending on the difference in polarityalignment of the incident pulse and the polarizing beam splitter, toform the first sub-pulse. The unreflected portion is transmitted intothe optical circuit as the second sub-pulse.

The second sub-pulse may be further subdivided to broaden the resultingrecombined pulse further. As the second sub-pulse approaches therecombination point where the first and second sub-pulses were firstdivided, differential polarity may again be employed to split the secondsub-pulse into further sub-pulses, one of which transits the opticalcircuit again. In this way an energy pulse with a long decay may beformed as a series of time overlapping, decaying amplitude, sub-pulses.

The temporal shape of the resulting pulse may be adjusted by adjustingthe relative phases of the differential polarity applicators withrespect to the polarity of the polarizing beam splitter. The length ofthe optical circuit may also be adjusted along with the polarizers toafford further control over the shape of the resulting pulse.

A train of regular energy pulses with simple periodicity may betransformed in complex ways by changing periodicity and amplitude of theresulting pulses by routing the pulses through a differential polaritydelay circuit as described herein. The fractions reflected andtransmitted at each beam splitter, and the length of the delay circuit,may be controlled relative to duration and periodicity of the incidentpulse train to produce a wide variety of complex patterns ofperiodicity, amplitude, and temporal profile.

A train of energy pulses having complex periodicity and amplitudepatterns, produced for example using a plurality of energy sources suchas switched lasers under the control of electronic timers, may be foldedinto a pulse train having a wide variety of characteristics from veryregular to extremely irregular depending on architecture and tuning ofthe optical circuitry used to subdivide and recombine the pulses.Depending on the interaction of the periodicity and amplitude pattern ofthe incident pulse train with the time constants of the optical circuitand the settings of the differential polarity splitters, a veryirregular train of pulses may be effectively regulated by tuning theoptical circuit to produce sub-pulses that substantially overlap in aregular manner.

FIG. 5 is a graph showing pulse intensity as a function of time for anenergy pulse processed using apparatus and methods of FIGS. 1-4. Thetemporal profile of the incident energy pulse is shown at 502, and thetemporal profile of the resultant energy pulse is shown at 504 afterpassing through a pulse width controller according to an embodimentdescribed herein. The amplitudes of the two profiles are not representedon the same scale, so that the general shape of each profile isdistinguishable from the other. The time evolution of each profile isplotted on the same scale, however, to demonstrate that the incidentpulse has been made longer in time by passing through a pulse widthcontroller according to an embodiment described herein.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. An optical apparatus, comprising: a first rotatable wave plate having an optical axis; a polarizing beam splitter with a reflecting axis and a transmitting axis, wherein the transmitting axis is substantially parallel to the optical axis of the first rotatable wave plate; a first reflector positioned along the transmitting axis and propagating a first deflected electromagnetic radiation along a first deflection axis; and a second reflector positioned to receive electromagnetic radiation originating from the first reflector and propagate a return electromagnetic radiation along the reflecting axis.
 2. The optical apparatus of claim 1, further comprising a second rotatable wave plate having an optical axis substantially parallel to the reflecting axis of the polarizing beam splitter.
 3. The optical apparatus of claim 1, wherein the second reflector is positioned along the first deflection axis.
 4. The optical apparatus of claim 1, further comprising a third reflector positioned along the first deflection axis and facing a second deflection axis, and the second reflector is positioned along the second deflection axis.
 5. The optical apparatus of claim 2, further comprising a first actuator coupled to the first rotatable wave plate and a second actuator coupled to the second rotatable wave plate, and a controller coupled to the first actuator and the second actuator.
 6. The optical apparatus of claim 2, further comprising a second polarizing beam splitter positioned along the first deflection axis, the second polarizing beam splitter having a second reflecting axis and a second transmitting axis, wherein the second reflector is positioned along the second reflecting axis.
 7. The optical apparatus of claim 6, further comprising a third wave plate positioned between the first reflector and the second polarizing beam splitter, a plurality of reflectors to receive a transmitted electromagnetic radiation along the second transmitting axis and direct the transmitted electromagnetic radiation along a circuit, and a fourth wave plate positioned in the circuit to receive the transmitted electromagnetic radiation and propagate a second return electromagnetic radiation along the second reflecting axis.
 8. The optical apparatus of claim 1, wherein the second rotatable wave plate is positioned between the second reflector and the polarizing beam splitter.
 9. A system for processing a substrate comprising: a source of electromagnetic energy; an optical system for focusing the electromagnetic energy; and a pulse width controller optically coupled to the source of electromagnetic energy and the optical system, the pulse width controller comprising two or more rotatable wave plates.
 10. The system of claim 9, wherein the two or more rotatable wave plates comprise a first wave plate and a second wave plate, and wherein a polarizing beam splitter is positioned on an optical axis from the first wave plate to the second wave plate.
 11. The system of claim 10, wherein the second wave plate is disposed in an optical circuit oriented to receive electromagnetic radiation from the polarizing beam splitter and propagate electromagnetic radiation to the polarizing beam splitter.
 12. The system of claim 11, wherein the source of electromagnetic energy comprises two or more lasers.
 13. The system of claim 12, wherein the pulse width controller further comprises a first actuator coupled to the first rotatable wave plate and a second actuator coupled to the second rotatable wave plate, and the system further comprises a controller coupled to the first actuator and the second actuator.
 14. The system of claim 13, further comprising a second polarizing beam splitter disposed in the optical circuit.
 15. The system of claim 11, wherein the first wave plate is disposed to deliver polarized radiation to the polarizing beam splitter.
 16. The system of claim 14, wherein the second rotatable wave plate is positioned to deliver polarized radiation to the second polarizing beam splitter.
 17. The system of claim 16, further comprising a third rotatable wave plate. 